1. Field of the Invention
This invention relates to a gas-detection sensor and more particularly to a solid state mass spectrograph which is micro-machined on a semiconductor substrate, and, even more particularly, to a preconcentrator array for concentrating the gas to be sampled by the mass spectrograph.
2. Description of the Prior Art
Various devices are currently available for determining the quantity and type of molecules present in a gas sample. One such device is the mass-spectrometer.
Mass-spectrometers determine the quantity and type of molecules present in a gas sample by measuring their masses. This is accomplished by ionizing a small sample and then using electric and/or magnetic fields to find a charge-to-mass ratio of the ion. Current mass-spectrometers are bulky, bench-top sized instruments. These mass-spectrometers are heavy (100 pounds) and expensive. Their big advantage is that they can be used in any environment.
Another device used to determine the quantity and type of molecules present in a gas sample is a chemical sensor. These can be purchased for a low cost, but these sensors must be calibrated to work in a specific environment and are sensitive to a limited number of chemicals. Therefore, multiple sensors are needed in complex environments.
A need exists for a low-cost gas detection sensor that will work in any environment. U.S. patent application Ser. No. 08/124,873, filed Sep. 22, 1993, hereby incorporated by reference, discloses a solid state mass-spectrograph which can be implemented on a semiconductor substrate. FIG. 1 illustrates a functional diagram of such a mass-spectrograph 1. This mass-spectrograph 1 is capable of simultaneously detecting a plurality of constituents in a sample gas. This sample gas enters the spectrograph 1 through dust filter 3 which keeps particulate from clogging the gas sampling path. This sample gas then moves through a sample orifice 5 to a gas ionizer 7 where it is ionized by electron bombardment, energetic particles from nuclear decays, or in a radio frequency induced plasma. Ion optics 9 accelerate and focus the ions through a mass filter 11. The mass filter 11 applies a strong electromagnetic field to the ion beam. Mass filters which utilize primarily magnetic fields appear to be best suited for the miniature mass-spectrograph since the required magnetic field of about 1 Tesla (10,000 gauss) is easily achieved in a compact, permanent magnet design. Ions of the sample gas that are accelerated to the same energy will describe circular paths when exposed in the mass-filter 11 to a homogenous magnetic field perpendicular to the ion's direction of travel. The radius of the arc of the path is dependent upon the ion's mass-to-charge ratio. The mass-filter 11 is preferably a Wien filter in which crossed electrostatic and magnetic fields produce a constant velocity-filtered ion beam 13 in which the ions are disbursed according to their mass/charge ratio in a dispersion plane which is in the plane of FIG. 1.
A vacuum pump 15 creates a vacuum in the mass-filter 11 to provide a collision-free environment for the ions. This vacuum is needed in order to prevent error in the ion's trajectories due to these collisions.
The mass-filtered ion beam is collected in a ion detector 17. Preferably, the ion detector 17 is a linear array of detector elements which makes possible the simultaneous detection of a plurality of the constituents of the sample gas. A microprocessor 19 analyses the detector output to determine the chemical makeup of the sampled gas using well-known algorithms which relate the velocity of the ions and their mass. The results of the analysis generated by the microprocessor 19 are provided to an output device 21 which can comprise an alarm, a local display, a transmitter and/or data storage. The display can take the form shown at 21 in FIG. 1 in which the constituents of the sample gas are identified by the lines measured in atomic mass units (AMU).
Preferably, mass-spectrograph 1 is implemented in a semiconductor chip 23 as illustrated in FIG. 2. In the preferred spectrograph 1, chip 23 is about 20 mm long, 10 mm wide and 0.8 mm thick. Chip 23 comprises a substrate of semiconductor material formed in two halves 25a and 25b which are joined along longitudinally extending parting surfaces 27a and 27b. The two substrate halves 25a and 25b form at their parting surfaces 27a and 27b an elongated cavity 29. This cavity 29 has an inlet section 31, a gas ionizing section 33, a mass filter section 35, and a detector section 37. A number of partitions 39 formed in the substrate extend across the cavity 29 forming chambers 41. These chambers 41 are interconnected by aligned apertures 43 in the partitions 39 in the half 25a which define the path of the gas through the cavity 29. Vacuum pump 15 is connected to each of the chambers 41 through lateral passages 45 formed in the confronting surfaces 27a and 27b. This arrangement provides differential pumping of the chambers 41 and makes it possible to achieve the pressures required in the mass filter and detector sections with a miniature vacuum pump.
Because of its size and power requirements, a micro-miniature mass-spectrograph 1 is already attractive as an integrated gas sensor. The detection sensitivity of such a device is projected to be limited to 0.1 parts per million (ppm). Many applications, from breath analyzing in the medical field to gas monitoring of the environment, require sensitivity in the parts per billion (ppb) range. This requires improving the sensitivity of the mass-spectrograph 1 without slowing down measurement speed or accuracy. The preferred integration time for a mass-spectrograph 1 is currently approximately 100 mseconds per window, translating to a total of approximately 2 seconds for scanning a mass range extending from 1 to 650 amu.
Improving sensitivity by a factor of 100 can be achieved by adding a chemical separator or preconcentrator as an input stage to the mass-spectrograph 1. However, preconcentrator absorption and desorption time should be kept to a minimum (ideally 2 seconds or less) for measurement timeliness. Furthermore, size and power must be minimized to maintain a high degree of portability.
Gas chromatographs can act as chemical separators. A gas sample is transported through a capillary tube via a carrier gas such as helium. Selective adsorption/desorption along the length of the tube results in separation of the gas sample's constituents. Detection is accomplished at the end of the tube as each constituent passes by, usually by measuring the gas's thermal conductivity. Gas chromatographs have been reduced in size to 3" diameter by 3/4" thick using micro-machining or micro-capillary technology. While the micro-machined or micro-capillary gas chromatograph is an attractive candidate for use with current mass-spectrometers, it is too large compared with micro-miniature mass-spectrographs (1 sq. in.times.0.030") and requires 5 to 8 watts for operation.
Preconcentrators have been used with surface acoustic wave chemical sensor arrays. Such preconcentrators consist of a 1.5" long glass tube with a 1/8" inner diameter packed with approximately 1/2 of 40-60 mesh Tenax. Such preconcentrators sorb in one direction and desorb in the other. A nichrome wire and thermistor are attached outside of the glass tube and are used to heat the preconcentrators to 200 degrees C during desorb. The current thermal desorbers used for preconcentration are large, cumbersome and require several watts.
The issue of input stage size and power, particularly for extended field operation, makes these methods of preconcentrators undesirable for a low power, handheld instrument. Accordingly, there is a need for an improved micro-miniature preconcentrator.